Bring get_code_dependency from teal.code to teal.data

[m7pr]

We once had get_code(qenv, names) in the prototype phase of code parser insightsengineering/teal.code#133.

If names were specified then get_code used get_code_dependencies. This was meant to work on qenv class, that's why it was developed in teal.code. However, we then removed it, so that we do not change too much in the user interface. We specifically left get_code_dependencies non-exported, so that it could be only used in teal.data::get_code.teal_data in the future.

If we plan to let it work only on teal_data which is introduced in teal.data I think it's fine enough to move get_code_dependencies from teal.code to teal.data.

Even though get_code_dependencies works on character input, it is designed to work on teal_data@code.

Potentially improve with #192